Add demo mode support with MSW integration and documentation

- Integrated Mock Service Worker (MSW) for frontend-only demo mode, allowing API call interception without requiring a backend.
- Added `DemoModeBanner` component to indicate active demo mode and display demo credentials.
- Enhanced configuration with `DEMO_MODE` flag and demo credentials for user and admin access.
- Updated ESLint configuration to exclude MSW-related files from linting and coverage.
- Created comprehensive `DEMO_MODE.md` documentation for setup and usage guidelines, including deployment instructions and troubleshooting.
- Updated package dependencies to include MSW and related libraries.

frontend/package.json

@@ -85,10 +85,16 @@
     "jest": "^30.2.0",
     "jest-environment-jsdom": "^30.2.0",
     "lighthouse": "^12.8.2",
+    "msw": "^2.12.3",
     "prettier": "^3.6.2",
     "tailwindcss": "^4",
     "typescript": "^5",
     "typescript-eslint": "^8.15.0",
     "whatwg-fetch": "^3.6.20"
+  },
+  "msw": {
+    "workerDirectory": [
+      "public"
+    ]
   }
 }